Cleaning up C++ input validation of strings

[Rik]

2/13/15

There seem to be multiple instances where we check whether an input is a
string, then extract a string_value, and then check the error_status to
make sure that the string_value was extracted correctly.  This seems
redundant and possibly a lazy copying of a correct pattern.

If something might not be of type A, but possibly could be converted to
type A, then asking for A_value () and checking the error_status is the
correct thing to do.  In this case, If something is determined to be a
string, can the string_value() call really fail?

Here is an example from lu.cc

-- Code --
if (args(n).is_string ())
  {
    std::string tmp = args(n++).string_value ();

    if (! error_state)
      {
        if (tmp.compare ("vector") == 0)
          vecout = true;
        else
          error ("lu: unrecognized string argument");
      }
  }
-- End Code --

I propose simplifying to

-- Code --
if (args(n).is_string ())
  {
    std::string tmp = args(n++).string_value ();

    if (tmp.compare ("vector") == 0)
      vecout = true;
    else
      error ("lu: unrecognized string argument");
  }
-- End Code --

Are there any objections?

--Rik